Cool game! I liked the concept of using your corpse as a platform to continue the level with (my group's game was originally going to be something similar to this), and the art and audio were really calming and helped me get to the end of the game (I did beat it without the key, I didn't even know there was one XD). For a platformer-builder hybrid, making the main character a cube was a genius idea due to the simplicity of the programming and gameplay, and the rest of the art style worked really well with it. The lighting around the cube was also a nice touch, giving the level and the world some life around the player.
The main issue for me was the level design, because it was really easy for me to get stuck in impossible situations and have to restart from the beginning, and that got frustrating after the fifth or sixth reset. An easy fix (I think) would've been having the reset button take you to the most recent checkpoint, but I can imagine how it might've been difficult to save all the corpses for that reset. Another option would be to place the kill barriers in more forgiving places as to not soft-lock the player, or even add a "kill" button so I could kill my character wherever and make a staircase upwards. To be fair, these suggestions would make the game easier, in a way, so if its current difficulty is your intention then maybe just give the player more room to put corpses and less corridors to get trapped in.
Also a small thing I didn't like was the way you could cling to the walls. Sometimes I wanted to just jump over a ledge, and if I touched a wall then I would lose all my upward momentum and start falling. This was particularly difficult since the movement sensitivity playing on keyboard was too high and I would easily slip further than I wanted. So I think the game could be better with slower horizontal speed and no sticking to walls. No need to touch the jumping and falling movement, though, you did a great job with that and it feels great in the game.
Overall, cool concept, smart character and art design, a little slippery with the horizontal movement, too sticky on the walls, and just a simple, fun experience. Nice job!